This invention relates to a martensitic stainless steel welded pipe excellent in corrosion resistance, in particular in stress corrosion cracking resistance. More particularly, it relates to a martensitic stainless steel large-diameter and thick-wall welded pipe exceeding 20 inches in outside diameter and exceeding 0.5 inch in wall thickness, which is used in a pipeline, in particular a trunk line, for conveying a fluid such as oil or natural gas readily corroding metals.
Large-diameter, thick-wall stainless steel pipes exceeding 20 inches in outside diameter and 0.5 inch in wall thickness are widely used in pipelines, in particular trunk lines, for conveying oils or natural gases which readily cause corrosion of metals.
Such large-diameter, thick-wall pipes are generally produced by a process which comprises forming a thick plate or hot strip into an open pipe or spiral form by bending and then welding the joining portions together. The steel pipes produced by such a process are called UO pipes or spiral pipes.
UO pipes are produced by a forming process comprising the steps referred to by the alphabetic letters (U and O) indicative of the name given thereto. In this process, a thick plate is formed into an open pipe form bent by using a U press, then the side edges are joined to each other to form a pipe shape by means of an O press and the joining portions are welded together.
Spiral pipes are produced by forming a hot strip into a pipe shape by spirally bending the hot strip in succession and then welding the joining portions together, side edge to side edge of the strip.
Processes other than those mentioned above are also available for the production of large-diameter, thick-wall pipes. For example, there is a process which comprises forming a thick plate into a pipe-like shape using a 3-roll type forming machine called roll bender and then seam-welding the side edge to side edge in order to join portions of the thick plate together.
In producing these large-diameter, thick-wall pipes, the submerged arc welding method (hereinafter referred to as xe2x80x9cSAW methodxe2x80x9d) is widely used. In producing large-diameter, thick-wall pipes by welding, one-layer welding is generally carried out from each of inside and outside of the pipe. Further, when the wall of the material is thick, multi pass (at least three) layer welding, in which two or more bead layers are formed from one or both sides, may be conducted in some instances.
Heretofore, in producing large-diameter, thick-wall welded pipes to be used in conveying fluids, such as oils and natural gases, readily causing corrosion of metals, steel plates made of carbon steel or low alloy steel containing at most about 1% by mass of Cr have been used as the base metals together with welding materials. The reason why large-diameter, thick-wall welded pipes made of carbon steel, which is inferior in corrosion resistance, have been used is that carbon steel is economically more advantageous. However, carbon steel is poor in corrosion resistance. Therefore, for pipelines constituted of welded pipes made of carbon steel, it has been a common practice to subject the crude oil or natural ngas obtained from an oil well to dehydration to thereby reduce the corrosiveness of the fluid.
However, the cost of construction of the dehydration equipment and the platform for the installation thereof is high. Therefore, the use of a more corrosion-resistant material has been begun for the production of pipes for pipelines while omitting the dehydration equipment. The material having higher corrosion resistance includes stainless steel.
In this case, neither dehydration equipment nor platform therefor is required at exploration locations and this fact is very advantageous to the exploration of small-scale oil or gas wells, for example horizontal wells, which cannot have been drilled. Specifically, it is an advantage that crude oils can be conveyed through such pipelines to an existing platform and collectively treated there for dehydration.
In high-latitude districts at north latitude 70xc2x0 or higher where future exploration is expected, for example oil wells in the North Sea, the platform construction itself is difficult from the viewpoint of waves on the sea. In that case, it is necessary to transport crude oils through pipelines without dehydration treatment.
With such background, large-diameter, thick-wall welded pipes enabling the omission of dehydration treatment are more and more desired for conveying fluids readily corrosive against metals.
Some stainless steels which are highly corrosion-resistant and suitable for conveying such fluids as mentioned above, and seamless pipes or electric resistance welded pipes or laser welded pipes made thereof with a relatively small diameter and a relatively thin wall, have been proposed. As for large-diameter, thick-wall welded pipes made of stainless steel, welded pipes and base metals therefor are disclosed in JP Kokai H10-60599 and JP Kokai H12-8144, for instance.
For the above application, martensitic stainless steel containing 9-13% by mass of Cr are used from the economical viewpoint. This is because martensitic stainless steel has, in addition to the economical feature, sufficient corrosion resistance under such circumstances as mentioned above, and is excellent in hot workability and, therefore, can readily be made into thick plates or hot-rolled plates, which are materials for the production of welded pipes.
It has been considered that these martensitic stainless steels used for base metals and the weld metals of seam weld portions are excellent in stress corrosion cracking resistance (hereinafter referred to as xe2x80x9cSCC resistancexe2x80x9d), carbon dioxide gas corrosion resistance (hereinafter referred to as xe2x80x9cCO2 resistancexe2x80x9d) and sulfide stress corrosion resistance (hereinafter referred to as xe2x80x9cSSC resistancexe2x80x9d or xe2x80x9csour gas resistancexe2x80x9d).
However, it has been revealed that when welded pipes made of martensitic stainless steel are used in a pipeline for conveying a corrosive fluid without dehydration, stress corrosion cracking (hereinafter referred to as xe2x80x9cSCCxe2x80x9d) tends to occur at the weld portion of the pipe inside surface. In particular, with large-diameter, thick-wall welded pipes produced by the SAW method without cutting off the weld bead on the pipe inside and outside surfaces, the tendency toward occurrence of SCC is significant. Furthermore, it has become apparent that even if the pipes have SCC resistance, the base metal and weld metal may be poor in sour gas resistance and the weld metal may be high in weld hot cracking susceptibility in some instances.
The object of the present invention is to provide a large-diameter, thick-wall martensitic stainless steel welded pipe excellent in corrosion resistance, in particular stress corrosion cracking resistance (SCC resistance), at the base metal portion and the seam weld portion of pipe inside surface and, further, excellent in sulfide stress corrosion resistance (sour gas resistance) and carbon dioxide gas corrosion resistance (CO2 resistance).
The stainless steel welded pipe of the invention is composed of a base metal which is a stainless steel containing not more than 0.05% by mass of C and 9-20% by mass of Cr and having metallurgical microstructures comprising a full martensite phase or a martensite phase as the main constituent with a ferrite phase contained therein, and a seam weld metal which is a stainless steel containing not more than 0.1% by mass of C and 7-20% by mass of Cr and having metallurgical microstructures comprising a martensite phase as the main constituent with an austenite phase contained therein. Further, the seam weld bead on the inside surface of the welded pipe of the invention satisfies the following relation (1):
Lxe2x89xa60.2xc3x97Wxe2x80x83xe2x80x83(1)
where
L: the length of the portions of the seam weld bead showing a value of h which exceeds 1.25 as calculated by the expression (2) shown below:
h={1+(2xc3x97H/W)}xc3x97(YSB100/YSw100)xe2x80x83xe2x80x83(2) 
where
H: the height of the bead from the pipe inside surface (mm),
W: the width of the bead (mm),
YSB100: the yield strength of the base metal at 100xc2x0 C. (MPa),
YSw100: the yield strength of the weld metal at 100xc2x0 C. (MPa).
As for the metallugical microstructures of the base metal and weld metal, those of the base metal are desirably constituted of 55-90% martensite phase and 10-45% ferrite phase and those of the weld metal are desirably constituted of 70-95% martensite phase and 5-30% austenite phase, on the volume % basis.
The contents of chemical components other than C and Cr of the base metal and of the weld metal of the seam weld portion of the welded pipe of the invention are desirably within the respective ranges shown below:
For the base metal, on the mass % basis:
For the weld metal, on the mass % basis:
For both the base metal and weld metal, the contents of P, S and O (oxygen) among the impurities are desirably as follows: P: not more than 0.025%, S: not more than 0.01% and O: not more than 0.01%, while the content of N is desirably not more than 0.02% for the base metal and not more than 0.05% for the weld metal.
The welded pipe mentioned above is a basic welded pipe of the present invention, and the welded pipe of the invention which satisfied the above conditions is excellent in SCC resistance, in particular. The base metal used for pipe production may be the one as hot rolled. Further, this pipe shows sufficient SCC resistance and CO2 resistance even when the heat treatment of the heat affected zone of the base metal and of the weld metal portion is omitted after welding.
The contents of Cr, Ni and Ti in the base metal of the basic welded pipe of the invention desirably fall within the respective ranges given below and further satisfy the relations (3) and (4) given below. In this case, even when the base metal is as hot rolled, it is excellent not only in SCC resistance but also in sour gas resistance and, even when the heat treatment of the weld portion after welding is omitted, these resistances are satisfactory.
Cr+1.5Moxe2x88x92Nixe2x88x920.4Cuxe2x88x9214xe2x89xa70xe2x80x83xe2x80x83(3) 
Cr+1.5Moxe2x88x922Nixe2x88x920.8Cuxe2x88x9212.5xe2x89xa60xe2x80x83xe2x80x83(4) 
For the above basic welded pipe, it is also desirable that the Cr, Ni, Mo and Ti contents of the base metal as shown below are combined with the Cr, Ni, Mo and Ti contents of the weld metal as shown below and that the chemical composition of the weld metal satisfy the relations (5) and (6) given below. In this case, even when the heat treatment of the weld portion after welding is omitted, the base metal and weld metal are excellent in toughness and strength and the weld metal portion is excellent with respect to sour gas resistance and weld hot cracking susceptibility.
xe2x88x921xe2x89xa6Cr+Moxe2x88x921.7Nixe2x89xa613xe2x88x92220xc3x97O (oxygen)xe2x80x83xe2x80x83(5) 
25xe2x89xa6Cr+Mo+1.8Nixe2x89xa630xe2x80x83xe2x80x83(6) 
In addition to the welded pipe mentioned above, the basic welded pipe according to the invention, when the base metal and seam weld metal have the chemical compositions respectively given below on the mass % basis, shows the highest SCC resistance.
The above-mentioned welded pipe of the invention is most suited for use as a pipe for a pipeline for conveying crude oil or natural gas without dehydration.
The invention is directed to a large-diameter, thick-wall welded pipe and the term xe2x80x9clarge diameterxe2x80x9d means not less than 20 inches (508 mm) and the term xe2x80x9cthick wallxe2x80x9d means not less than about 0.5 inch (12.7 mm). The term xe2x80x9cmartensite phase as the main constituentxe2x80x9d means that the martensite phase proportion exceeds 50% by volume.